The emails, snatched by hackers, could unmask sensitive sources and throw light on the murky world of intelligence-gathering by the company known as Stratfor, which counts Fortune 500 companies among its subscribers.
Stratfor in a statement shortly after midnight EST (0500 GMT) said the release of its stolen emails was an attempt to silence and intimidate it.
It said it would not be cowed under the leadership of George Friedman, Stratfor's founder and chief executive officer. It said Friedman had not resigned as CEO, contrary to a bogus email circulating on the Internet.
Some of the emails being published "may be forged or altered to include inaccuracies; some may be authentic," the company statement said.
"We will not validate either. Nor will we explain the thinking that went into them. Having had our property stolen, we will not be victimized twice by submitting to questioning about them," the statement said.
Hackers linked to the loosely organized Anonymous hackers group said at the beginning of the year they had stolen the email correspondence of some 100 of the firm's employees. The group said it planned to publish the data so the public would know the "truth" about Stratfor operations.
Stratfor describes itself as a subscription-based publisher of geopolitical analysis with an intelligence-based approach to gathering information.
WikiLeaks and Anonymous maintain the emails will expose dark secrets about the company. Stratfor said in its statement it had worked hard to build "good sources" in many countries, "as any publisher of global geopolitical analysis would do."
In December, hackers broke into Stratfor's data systems and stole a large number of company emails.
WikiLeaks founder Julian Assange told Reuters: "Here we have a private intelligence firm, relying on informants from the U.S. government, foreign intelligence agencies with questionable reputations and journalists."
"What is of grave concern is that the targets of this scrutiny are, among others, activist organizations fighting for a just cause."
Friedman, the chief executive, said on Jan. 11 the thieves would be hard pressed to find anything significant in the stolen emails.
"God knows what a hundred employees writing endless emails might say that is embarrassing, stupid or subject to misinterpretation. ... As they search our emails for signs of a vast conspiracy, they will be disappointed."
People linked to Anonymous took credit for the data theft. "Congrats on the amazing partnership between #Anonymous and #WikiLeaks to make all 5 million mails public," AnonSec Tweeted. AnonSec is one of several Twitter accounts used to promote and organize activities associated with Anonymous.
It was not immediately clear what impact the release of the emails might have on Stratfor, its employees, clients and information sources.
Previous releases from WikiLeaks, such as secret video battle footage and thousands of U.S. diplomatic cables about the wars in Iraq and Afghanistan, in 2010 have angered the U.S. government. WikiLeaks' disclosures also have raised questions about the safety of confidential sources quoted in previously secret documents.
WikiLeaks said it was working with two dozen media organizations worldwide that have access to a database of the Stratfor emails. These include the U.S. newspaper publisher McClatchy Co..
"We have begun reviewing the emails and will publish as warranted," McClatchy's Washington bureau chief, James Asher, told Reuters.
The group gave a sneak preview of the emails to The Yes Men, an activist group that targets what it views as corporate greed.
"What is significant is the picture it helps to paint of the way corporations operate," Bichlbaum told Reuters. "They operate with complete disregard for rule of law and human decency."
After Stratfor's computers were hacked at least twice last December, the credit card details of more than 30,000 subscribers to Stratfor publications were posted on the Internet, including those of former U.S. secretary of state Henry Kissinger and former U.S. vice president Dan Quayle.
The FBI began investigating the matter in December.